The present invention relates to a method of resistance welding in which the welding electrode movement takes place at least by means of one working piston-cylinder unit.
Furthermore, the invention relates to a resistance welding plant with a working piston-cylinder unit for carrying out an electrode movement during the welding process.
Resistance welding is a generally known method in which, for example, two pieces of sheet metal are connected together, in that they are pressed together by two electrodes, of which at least one is movable, and fusing and subsequent solidification of the material takes place at the joint location by means of adequate electrical resistance heating through the welding current.
The quality of the welded connection is decisively influenced by how good the contact is between the electrode and the workpiece, because the contact resistances and thus the distribution of the heat produced are influenced by this contact.
The manner in which the electrode is placed onto the workpiece is thus, first of all, of significance, because if the placement movement is too rapid, the electrode rebounds so that the contact pressure force initially oscillates and the decay of this oscillation must be waited for prior to switching on the welding current, which, however, reduces the working speed in a disadvantageous manner. During spot welding rapid placement can lead to an uncontrolled high striking energy, which leads to cold deformation and rapid wear of the electrodes.
During projection welding the too rapid placement brings about an uncontrolled cold deformation of the projection prior to switching on of the welding current, and thus to an uncertain welding result.
One has hitherto attempted to regulate the speed of placement through the association of damping and restrictor valves. One has achieved a certain improvement of the placement behavior through the use of double stroke and feed stroke units.
The force responsible for the contact pressure between the electrode and the workpiece must, however, not only be made available at the start of welding but rather also during welding.
Of significance in this connection is the fact that the material to be welded becomes soft during the welding, i.e. pasty, and fuses at the joint position. When the electrode cannot be adequately quickly replenished, the force between the workpiece and the electrodes can, in unfavorable cases, reduce to such an extent that no adequate electrical contact is any longer present between the electrodes and the workpiece. This leads, having regard to the extremely high current density, to disadvantageous spray formation and premature electrode wear. If, during projection welding, the electrode is not adequately rapidly replenished, the contact pressure between the impressed projection of the one piece of sheet metal and the counter-sheet reduces, whereby the softened projection can spray away or pore formation can arise in the welding spot.
In the previously known methods, the electrode force is primarily made available during welding by single stroke cylinders, or by the working stroke of twin or feed stroke cylinders, which are, as a rule, pneumatically actuated.